Fix HighlightTrigger.background_color property getter signature

[mkusaka]

Summary

Removed a stray value parameter from the HighlightTrigger.background_color @property getter. Accessing trigger.background_color raised TypeError because Python's property descriptor calls the getter with only self.

Background

• The getter signature was copy-pasted from the setter below it, leaving the value: typing.Optional[iterm2.Color] parameter in place
• The corresponding text_color property does not have this issue
• Also added the missing return type annotation (-> typing.Optional[iterm2.Color]) to match the text_color getter

Example

trigger = HighlightTrigger(regex="error", text_color=None, background_color=None, instant=False, enabled=True)

trigger.text_color        # OK — returns None
trigger.background_color  # TypeError: background_color() missing 1 required positional argument: 'value'

Verification

Confirmed that trigger.background_color returns the expected value without TypeError, and mypy --ignore-missing-imports iterm2/triggers.py no longer reports Too many arguments for property [misc] on line 713.